EP 1 965 483 A1 discloses a supplementary device that is to be arranged between a public AC grid and a standalone generating installation. The supplementary device has an independent disconnection device with which the standalone generating installation can be connected to the AC grid. A grid monitoring device situated in the supplementary device opens the independent disconnection device in the case of grid failure of the public AC grid. Via a second switching element of the supplementary device, the standalone generating installation can be connected to an island grid with a plurality of loads connected to the supplementary device and an island inverter. The island inverter is fed from a battery. The island grid is also connected to the public electricity grid via a switch in the normal case, such that the electricity grid predefines an AC voltage in the island grid. Only if the AC grid fails the island grid is disconnected from the AC grid. Then the island inverter predefines the AC voltage in the island grid. The standalone generating installation can be connected to the AC voltage by means of the second switching element. If the island grid is connected to the public AC grid and is supplied from the latter, the battery of the island inverter is continuously recharged, such that enough energy is available in the event of failures of the AC grid. In this case, the energy flow and the voltage of the battery can be influenced in such a way as to ensure a long lifetime of the battery.
The final report “Modularer Batteriestromrichter: Entwicklung eines Batterie-stromrichters für die modulare Systemtechnik in PV-Anlagen” [“Modular battery converter: development of a battery converter for modular systems engineering in PV installations”], funded by the German Federal Ministry of Education, Science, Research and Technology, funding code: 0329741, discloses control of an island inverter depending on the state of charge of its battery. The state of charge is subdivided into three working ranges “battery normal”, “battery low” and “battery overloaded”. The working range “battery normal” comprises sub-ranges “N” having a state of charge of between 60% and 100%, “V1” having a state of charge of between 40% and 60% and “V2” having a state of charge of between 20% and 40%. In the range V2 of low availability of the battery, a diesel generator for charging the battery is started if the diesel generator is present in the island grid. The working range “battery low” is divided into a sub-range “A1” having a state of charge of 10% to 20%, a sub-range “A2” having a state of charge of 0% to 10%, and a sub-range “A3” having a state of charge of less than 0%. In the sub-range A1, the loads in the island grid are partly switched off. In the sub-range A2, all loads in the island grid are switched off. In the sub-range A3, the island inverter is also switched off in order to prevent a deep discharge of the battery. The working range “battery overloaded” is subdivided into two “sub-ranges “UE1” and UE2” with increasing overcharging of the battery. In the first sub-range UE1, additional switchable loads are switched on. In the event of the battery voltage rising further into the range UE2, electricity feeders such as photovoltaic and wind power installations connected to the island grid are limited or switched off.
Batteries are also used as buffering storage devices for electrical energy obtained from regenerative primary energies, in order to increase a local self consumption of the electrical energy at the location of its generation. In this case, electrical power is fed to the respective battery via a battery converter for as long as it is able to store electrical power if there is an excess of locally generated electrical power relative to the local power demand. Conversely, electrical power is drawn from the battery for as long as it can be made available by the battery if the locally generated electrical power does not cover the local power demand. In order to prevent the battery from a deep discharge, the battery is kept at a minimum state of charge. Self-discharge of the battery is compensated for by the supply of electrical power, which is drawn from a connected AC grid if necessary.